Some deep-drawn thermoformed decorative films -- such as may be used as an exterior covering on a furniture piece, desk accessory, automobile interior fitting, or the like -- carry a layer of pressure-sensitive adhesive for adhering the film to a configured substrate. Providing a release liner for such a decorative film that will cover and protect the layer of pressure-sensitive adhesive prior to, during, and after the thermoforming operation is a difficult challenge, requiring a sheet materal having an unusual combination of physical properties.
For example, the release liner must elongate and conform generally the same amount as the decorative film elongates and conforms during the thermoforming operation (in a deep-drawn thermoforming operation the film is generally elongated at least 50 percent during the operation, and parts of the film may be drawn larger amounts, so as to take the form of a particular detail or to be drawn into a sharp corners). Despite elongating to this extent during the thermoforming operation, the release liner must remain a continuous protective sheet that preferably can be removed as a single piece. Maintaining the integrity of the release liner is complicated by the fact that the heat for the operation is applied through the liner, so that the liner experiences high temperatures for a significant part of the operation.
A further requirement for base sheet materials to be used in a described release liner is that they be capable of being treated to provide preferred release properties. For example a release property is often provided by curing a silicone-based release treatment on a base sheet material at temperatures of 150.degree.F or more. Many polymeric films will elongate under tension so easily at those temperatures that they cannot be handled on continuous process equipment.
Polyethylene and polypropylene are commonly used as release liners, but they are not fully satisfactory for the described thermoformable pressure-sensitive-adhesive-covered decorative films. One deficiency is that they must rupture or open during a thermoforming operation, so as to expose the pressure-sensitive adhesive layer and make it difficult to remove the liner in one piece. In addition, polypropylene tends to become brittle during the thermoforming operation, with the result that the liner may shatter to prevent easy removal, especially after die-cutting operations to remove flashing or unneeded sections from the thermoformed part.
Insofar as known, no one has ever provided release liners prior to the present invention that have the necessary properties for use on pressure-sensitive-adhesive-covered decorative films that are to be subjected to a deep-draw thermoforming operation. The absence of such a liner has been an obstacle to broad use of thermoformed, pressure-sensitive-adhesive-covered, decorative films.